Mass storage for a computer system is typically provided by an attached disk drive. Disk drives having size of 200 GB or more are increasingly common in desktop and laptop computer systems. Fast and efficient access to data stored on such drives is important to responsiveness and functionality of typical user applications. A typical disk drive comprises a motor to rotate the disk at a constant rate and one or more read/write heads which are positioned over a desired track on the disk by a servo mechanism. The disk drive also contains electronics to amplify the signals from the one or more heads and convert them to normal computer system digital logic levels and vice versa.
The disk surface is divided into concentric tracks (e.g., circles within circles), and data bits are magnetically recorded on the tracks. Modern high-capacity disk drives pack the bits as tightly as possible within each track. Tracks are further divided into sectors, which generally hold the least amount of data that can be read or written at one time. The disk drive initiates a read or write to a given location by positioning a read/write head radially over the right track and rotationally over the start of the right sector. For example, in order to update the disk, one or more sectors are read from the disk into the computer, changed and written back to disk.
Seek time and rotational latency comprise the two main components of data access latency for a disk drive. There is usually one head for each disk surface (e.g., platter) and all heads move together. The set of locations which are accessible with the heads in a given radial position are known as a “cylinder”. The “seek time” is the time taken to seek to a different cylinder. In most circumstances, the disk is constantly rotating, so positioning the heads over the right sector is simply a matter of waiting until it rotates under the head. With a single set of heads this “rotational latency” will be on average half a revolution. Access time is generally the sum of the seek time component plus the rotational latency component.
Modern hard drives reduce rotational latency by spinning the disk at very high rpm. Since rotational latency directly affects the access time for data on the hard drive, a brute force method to reduce access time is to simply spin the hard drive at a higher rpm. For example, a disk drive spinning at 10,000 rpm will have a lower rotational latency than a disk drive spinning at 7200 rpm. Decreasing rotational latency by simply increasing disk rpm, however, becomes increasingly expensive. High-speed drive mechanisms are very sensitive and difficult to manufacture. Furthermore, high-speed drive mechanisms generate an inordinate amount of heat, cause unwanted vibration, and decrease reliability of the disk drive. Thus, a new approach for reducing the time required to retrieve a block of data stored on the hard drive (e.g., beyond merely increasing rotation rpm) is required.
The SATA2 (Serial ATA version 2) specification specifies a non-zero offset approach to reduce rotational latency for a new architecture of disk drives. The SATA2 specification comprises a new specification designed to improve the performance of disk drives. SATA2 is designed to add features that improve the data transfer speed and efficiency of SATA2 disk drives in comparison to ATA (Advanced Technology Attachment) disk drives. Generally, the SATA2 non-zero offset approach reduces rotational latency by enabling a SATA2 disk drive to read the middle of a sequential series of sectors (e.g., sectors 18-31 out of 1-31) as they pass under the disk drive's read/write head and subsequently, read the earlier sectors (e.g., sectors 1-17) of the series of sectors after a rotation of the disk brings them under the read/write head.
A significant problem with this SATA2 specification non-zero offset approach is that it only applies to new architecture disk drives manufactured in accordance with the SATA2 specification. The built-in architecture level non-zero offset approach can only be implemented with hard drives manufactured in accordance therewith. The SATA2 non-zero offset approach does nothing to reduce rotational latency for the millions of non-SATA2 disk drives that are still being manufactured, already on the market, or in the supply chain. Thus, for these “legacy” disk drives, a new approach for reducing rotational latency is required.